Method of and apparatus for heating of dielectric materials in a microwave field

ABSTRACT

A method of and apparatus for heating, particularly dielectric materials by exposing them to the electromagnetic energy of a microwave field in a waveguide which is energized by a microwave generator and short-circuited at one end, which comprises the steps of heating dielectric material in a waveguide made of a solid dielectric nonmetallic material and containing a channel extending through its entire length. The channel is used for shaping and calibrating the dielectric material during the process of heating, continuously conveying axially through the waveguide in the direction of energy flux the dielectric material to be heated, and shielding the waveguide against radiant heat loss.

United States Patent Inventors Fritz Wiegmann Dresden; Hans Wagner, Sebnitz, both of, Germany AppL No. 856,789 Filed Sept. 10, 1969 Patented July 20, 1971 Assignee Sachsische Glashser-Industrie Wagner &

Co. K.G. Sebnitz, Germany METHOD OF AND APPARATUS FOR HEATING 0F DIELECTRIC MATERIALS IN A MICROWAVE FIELD Primary EQraminer-J V. Truhe Assistant Examiner-L. H. Bender Attorney-Emest G. Montague ABSTRACT: A method of and apparatus for heating, particularly dielectric materials by exposing them to the electromagnetic energy of a microwave field in a waveguide which is energized by a microwave generator and short-circuited at one end, which comprises the steps of heating dielectric material in a waveguide made of a solid dielectric nonmetallic material and containing a channel extending through its entire length. The channel is used for shaping and calibrating the dielectric material during the process of heating, continuously conveying axially through the waveguide in the direction of energy flux the dielectric material to be heated, and shielding the waveguide against radiant heat loss.

5 Claims, 2 Drawing Figs.

u.s.c|. 219/1055, 219/1065 1m.c|. 1105b 9/06, 1105b 5/00 FieldoiSearch 219/1055, 10.65

METHOD OF AND APPARATUS FOR HEATING F DIELECTRIC MATERIALS IN A MICROWAVE FIELD The present invention relates to improvements in or relating to the heating of dielectric materials in a microwave field, particularly by a microwave generator short-circuited at one end.

The present invention is more specifically applicable to the heating or hardening of rod-shaped, strip-shaped or bandshaped deformable plastic materials.

Particularly for heating dielectric materials several methods are already in use. One method consists in introducing or conveying the material that is to be heated into or through a metallically closed cavity which operates as a resonator, and which is supplied with energy by a microwave generator, the material accepting energy from the standing electromagnetic field in the interior of the cavity and being thus heated. The transfer of energy and the heating effect become less, when the resonator is not in tune with the frequency of the generator because of unavoidable irregularities in the dielectric pro perties and conductance of the material that is to be heated or because these change while the process of heating takes place. This difficulty arises particularly when low loss materials are heated and must be compensated by the resonator or the generator being retuned or by other control measures being taken. Particularly in large microwaveguides this involves a considerable expenditure in means besides creating major difficulties. This method is therefore primarily used for the batchwise heating of materials in a continuously repeated working cycle.

It has also been proposed to convey the material that is to be heated through a metallic tube which functions as a waveguide, and which is fed by a microwave generator, energy being abstracted from the traveling microwave field to heat the dielectric material. In the practical performance of this method use is made of arrangements in which a strip-shaped dielectric material of rectangular section placed across the axis of a waveguide is conveyed through a point of maximum field strength in the waveguide, the thickness of the material being small in relation to the dimensions of the waveguide cross section. Arrangements are also known in which a filamentous material is conducted axially through a waveguide of circular cross section, the gauge of the filament again being small in relation to the cross-sectional dimensions of the waveguide. Other arrangements known in the art avail themselves of a coiled guide instead of a metallic tube and a rodshaped dielectric material is conveyed through the microwave field along the axis of the coil.

The shortcomings of the above-described arrangements which use metal tubes or coil-shaped guides reside in that the thickness of the material that is to be heated must be small in relation to the cross sections of the waveguide or at least smaller than these, and in that the heated material may change its dimensions and shape during the process of heating because of thermal expansion or of physicochemical events that take place in the interior of the material. This is more particularly the case when the material that is to be heated is introduced into the heating zone in a liquid or viscous state and liquid or gaseous substances escape from the material during the process of heating.

It is one object of the present invention to provide the heating of dielectric materials in a microwave field, wherein the above-mentioned defects are eliminated.

The problem contemplated by the present invention is therefore that of heating dielectric materials of any desired cross sections in such a way that after having been heated the material will have and will retain a prescribed shape as well as prescribed dimensions, and that the arrangements used for the purpose can be incorporated in an industrial process of maximum productivity.

It is another object of the present invention, to provide the heating of dielectric materials in a microwave field, wherein the heating of the dielectric material in a waveguide made of a solid dielectric nonmetallic material and containing a channel extending through its entire length for conveying therethrough the dielectric material that is to be heated, is provided, and the channel being used for shaping and calibrating the dielectric material during the process of heating, and by continuously conveying the dielectric material that is to be heated through the waveguide in the direction of energy flux, the waveguide being shielded against radiation loss of electromagnetic ener- 8)- ln apparatus suitable for performing the method according to the present invention the length of the waveguide is preferably so chosen, that the electromagnetic energy that arrives at the short-circuited end of the waveguide and that has not been converted into heat in the dielectric material during its passage through the waveguide is reflected at the short-circuited end and passes through the waveguide a second time to be so attenuated by further heating of the dielectric material that the feeding microwave generator is not adversely affected or damaged.

The present invention permits, particularly plastics materials or resins to be heated, hardened and processed in an industrial process of considerable usefulness. The present invention is capable, with regard to the heating rate and the quality of heating that can be achieved, of exceeding the performance of conventional arrangements in which the material that is to be heated is conducted through a metal tube which is supplied with heat from a heated liquid bath by thermal conduction.

With these and other objects in view, which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawing, in which FIG. 1 is a schematic longitudinal section of apparatus for performing the method according to the present invention, and

H6. 2 discloses four cross sections taken on the lines 2-2 of H6. 1 of several illustrative cross sections of the waveguide and the channel which it contains.

Referring now to the drawing, channel 2 in a waveguide 1 made of a solid dielectric nonmetallic material extends through the waveguide from end to end. At one end of the waveguide l are coupling means 3 for coupling the electromagnetic energy supplied by a microwave generator 4 into the waveguide. The dielectric material that is to be heated is conveyed through the channel 2. At the end remote from the coupling means 3 the waveguide l is short-circuited at 6. A shield 7 prevents electromagnetic energy from being lost by radiation from the waveguide 1.

The material 5 that is to be heated is continuously conveyed axially through the waveguide l in the direction of the energy flux. The channel may have any desired cross section. Moreover, the cross section of the waveguide l itself is also arbitrary. During the process of heating the channel 2 operates to shape and calibrate the plastic dielectric material that is at first still in deformable state.

The length of the waveguide l is so chosen that the electromagnetic energy which arrives at the short-circuited end 6, and which has not been converted, during its passage through the waveguide l to heat in the dielectric material 5 is reflected at the short circuit 6 (short-circuited end) and propagated through the waveguide a second time to be sufficiently attenuated by further heating of the dielectric material 5 to prevent the feeding microwave generator 4 from being adversely affected or damaged.

While we have disclosed several embodiments of the present invention, it is to be understood that these embodiments are given by example only and not in a limiting sense.

We claim:

I. A method of heating, particularly dielectric materials by exposing them to the electromagnetic energy of a microwave field in a waveguide which is energized by a microwave generator and short-circuited at one end, comprising the steps of heating by microwave energy dielectric material in a waveguide made of a solid dielectr icnonmetallic material and containing a channel extending through its entire length, i

using said channel for shaping and calibrating the dielectric material during the process'of heating, and passing said dielectric material to be heated through said channel so 5 as to fill said channel in cross section,

' passing microwave energy from said microwave generator into said waveguide and through said waveguide in a first direction, said microwave energy reaching said short-circuited one end, reflecting said microwave energy reaching said short-circuited one end back through said waveguide, continuously conveying axially through the waveguide in said first direction of energy flux said dielectric material to be heated, l5 attenuating said reflected microwave energy by further heating therewith said dielectric material in a direction reverse to said first direction sufi'iciently such that said microwave generator is not adversely affected, and shielding said waveguide against radiant heat loss. 2. An apparatus for heating, particularly dielectric materials by exposing them to the electromagnetic energy of a microwave field in a waveguide which is energized by a microwave generator comprising a waveguide made of a solid dielectric nonmetallic material and having a channel extending through its entire length for continuously conveying therethrough a dielectric material to be heated passing therethrough filling the cross section of said channel,

means for coupling microwave energy from said microwave generator into said waveguide adjacent one end thereof,

first conduit means for passing said dielectric material to be heated into said one end of said waveguide and into said channel thereof, said waveguide being short-circuited at its other end and oriented to reflect microwave energy reaching said other end back through said waveguide,

second conduit means communicating with said waveguide channel at said other end for removing said dielectric material from said waveguide, and

the length of said waveguide being such, that the electromagnetic energy which arrives at the short-circuited end of said waveguide and which has not been converted to heat in the dielectric material during its passage through the waveguide is reflected at said short-circuited end and passes through said waveguide a second time to be sufficiently attenuated by further heating of said dielectric material that the feeding microwave generator is not adversely affected or damaged.

3. The apparatus, as set forth in claim 2, further comprising a shield means disposed about said wave guide for shielding said waveguide against radiant heat loss.

4. The apparatus, as set forth in claim 3, wherein said waveguide is straight in length,

said coupling means is radially oriented to radially direct incoming microwave energy into said one end of said waveguide, and

said short-circuited end comprises a plate disposed perpendicularly to the length of said waveguide.

5. The apparatus, as set forth in claim 4, wherein said plate has an outer periphery extending to the outer periphery of said shield means, and

said plate has an inner opening complementary to said waveguide channel. 

1. A method of heating, particularly dielectric materials by exposing them to the electromagnetic energy of a microwave field in a waveguide which is energized by a microwave generator and short-circuited at one end, comprising the steps of heating by microwave energy dielectric material in a waveguide made of a solid dielectric nonmetallic material and containing a channel extending through its entire length, using said channel for shaping and calibrating the dielectric material during the process of heating, and passing said dielectric material to be heated through said channel so as to fill said channel in cross section, passing microwave energy from said microwave generator into said waveguide and through said waveguide in a first direction, said microwave energy reaching said short-circuited one end, reflecting said microwave energy reaching said short-circuited one end back through said waveguide, continuously conveying axially through the waveguide in said first direction of energy flux said dielectric material to be heated, attenuating said reflected microwave energy by further heating therewith said dielectric material in a direction reverse to said first direction sufficiently such that said microwave generator is not adversely affected, and shielding said waveguide against radiant heat loss.
 2. An apparatus for heating, particularly dielectric materials by exposing them to the electromagnetic energy of a microwave field in a waveguide which is energized by a microwave generator comprising a waveguide made of a solid dielectric nonmetallic material and having a channel extending through its entire length for continuously conveying therethrough a dielectric material to be heated passing therethrough filling the cross section of said channel, means for coupling microwave energy from said microwave generator into said waveguide adjacent one end thereof, first conduit means for passing said dielectric material to be heated into said one end of said waveguide and into said channel thereof, said waveguide being short-circuited at its other end and oriented to reflect microwave energy reaching said other end back through said waveguide, second conduit means communicating with said waveguide channel at said other end for removing said dielectric material from said waveguide, and the length of said waveguide being such, that the electromagnetic energy which arrives at the short-circuited end of said waveguide and which has not been converted to heat in the dielectric material during its passage through the waveguide is reflected at said short-circuited end and passes through said waveguide a second time to be sufficiently attenuated by further heating of said dielectric material that the feeding microwave generator is not adversely affected or damaged.
 3. The apparatus, as set forth in claim 2, further comprising a shield means disPosed about said wave guide for shielding said waveguide against radiant heat loss.
 4. The apparatus, as set forth in claim 3, wherein said waveguide is straight in length, said coupling means is radially oriented to radially direct incoming microwave energy into said one end of said waveguide, and said short-circuited end comprises a plate disposed perpendicularly to the length of said waveguide.
 5. The apparatus, as set forth in claim 4, wherein said plate has an outer periphery extending to the outer periphery of said shield means, and said plate has an inner opening complementary to said waveguide channel. 